As you prepare your nutrient management plan for the 2023 growing season, don’t overlook the importance of pH and soil nutrients.

Soil is comprised of organic matter, minerals, water, air, and organisms on the Earth’s surface that enable farmers to plant, manage, and harvest high-yielding corn and soybeans every year. Soil is the medium that allows seeds to germinate, emerge, take root, and develop into plants. Productive soil has appropriate soil tilth, drainage, microbial activity, and nutrient availability to support plant growth.

Corn and soybean nutrient availability are mainly dependent on soil pH, as 14 of the 17 essential elements for crop growth are derived from the soil. Chlorine and nickel are relatively unaffected by soil pH, and carbon, hydrogen, and oxygen are obtained from the air and water.

A soil pH level is a component of all standard soil sample tests and measures the potential (p) for hydrogen ion (H+) and hydroxyl ion (OH-) concentration in water. Soil pH is presented on a 14-point logarithmic scale (a pH of 6 is 10 times more acidic than pH of 7), with 7.0 being neutral.

Values lower than 7.0 indicate an increase in the H+ ion concentration and acidity. Values higher than 7.0 increase the hydroxyl (OH-) ion concentration and alkalinity (pH > 7). Soil pH is dependent on many factors, including parent material, rainfall, and even agricultural practices. What does this have to do with nutrient availability? 

Our soil pH greatly impacts the solubility of minerals or nutrients and, therefore, the availability of plant uptake. Restricted root growth can compound the problem as the plant is unable to reach enough of the soil profile to compensate for reduced nutrient availability.

Commercial fertilizers and natural soil biology will not be as effective if the pH is not managed. Figure 1 highlights the influence of pH on soil nutrient availability. The optimal pH for plant growth is dependent on the crop, but the best soil pH for corn and soybeans would be between 6.0 to 6.5.

In acidic soils with a pH less than 6.0, the availability of nitrogen (N), phosphorus (P), potassium (K), sulfur (S), calcium (Ca), and other nutrients is reduced. Not only are these nutrients chemically less available at lower pH levels, but the microbial activity is diminished. This can slow down soil processes like the nitrogen cycle.

The availability of nutrients like iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), and aluminum (Al) increases in acidic soils. But the increases could be at levels that are toxic to the plant, especially in the case of Al, Fe, and Mn.

 in acidic soils can be corrected by applying lime. A good agricultural liming material neutralizes soil acidity with the help of Ca and/or Mg by replacing H+ ions and, consequently, raising the soil pH. Not all lime is created equal; most soil testing labs will analyze lime to determine its neutralizing value.

Growers may start to see benefits from lime during the first few months after application, but it will take two to three years for it to react completely with the soil. According to Michigan State University, increasing your soil pH from 5.7 to 6.5 in mineral soils can improve corn and soybean yields by 20% or more.

High pH corn and soybean soils often have a reduced availability of P, Zn, Fe, Cu, Boron (B), and Mn, which results in stunted plant growth. In general, as pH rises, most micronutrients decrease in availability apart from molybdenum. Alkaline soils typically have higher levels of Ca, Mg, and sodium (Na). The majority of high-pH soils contain calcareous materials such as limestone and are often found in arid crop areas like the Great Plains.

Unfortunately, correcting alkaline soils is no easy task. Soil pH can be reduced by applying elemental S, but it's generally not economically feasible.

The best option may be adding organic matter via manure applications or increased crop residue. However, this can take years to impact pH and, subsequently, crop performance. It is best to focus on hybrid and variety selection with good tolerance to high pH soils.